Vascular inflammation plays a key role in diseases such as atherosclerosis. Vascular inflammation consists of several discrete stages, and the first stage is driven by endothelial cell exocytosis. The goal of this application is to identify novel cellular factors that regulate endothelial exocytosis. Vesicle trafficking is a process of fundamental importance to cell biology. During exocytosis, vesicles deliver proteins from the Golgi to the cell exterior. Several families of proteins have been identified as mediating exocytosis, but our knowledge of the exocytic machinery is fragmentary and incomplete. We do not know: (1) how cells regulate the number of transport vesicles, (2) how budding vesicles separate from the donor compartment, or (3) how vesicles are targeted to their correct destination. Identification of novel proteins in the exocytic pathway may help answer some of these fundamental questions. A critical barrier to progress in this field lies in historical experimental models. Most of our knowledge about transport relies on the study of neurons secreting neurovesicles. However, exocytosis of neurovesicles is challenging to study: assays for neurotransmitters are neither rapid nor robust. In contrast, endothelial cells secrete von Willebrand Factor into the media, a protein easy to measure in a high-throughput format. We hypothesize that a discrete set of proteins regulates exocytosis of endothelial granules. To test this hypothesis, we plan to identify and characterize proteins involved in exocytosis, in the following specific aims: Aim #1: Identify novel proteins that regulate endothelial exocytosis by a functional genomic screen. We will perform a large-scale RNA interference screen to identify proteins and pathways that modulate exocytosis in endothelial cells. After excluding genes that affect cell viability, we will validate individual genes that increase or decrease endothelial exocytosis. Aim #2: Characterize the role of novel proteins that regulate exocytosis in endothelial cells. Next we will characterize a select few of the candidate gene products identified in Aim #1. We will determine which stage of vesicle trafficking these candidate proteins affect. We will study the role of candidate gene products in granule biogenesis, cargo loading, vesicle trafficking, and membrane fusion. In addition, we will also study 5 candidate genes previously identified in a genome wide association study by our Consultant which are associated with altered endothelial exocytosis in humans. The significance of this proposal is that vesicle trafficking pathways are important but not well understood, and they are critical to human vascular inflammatory diseases such as atherosclerosis. The innovation of this proposal is (1) identification of novel genes that regulate vesicle trafficking, (2) characterization of novel gene products associated with exocytosis recently identified in humans, (3) new approaches to identifying exocytic proteins in human cells.